Fixed warning messages when auto-registering a new LDAP user.

On my test system (without memcache), while testing the LDAP
authentication plugin, when I sign in for the first time, triggering
auto-registration, I get these messages in the output page:

Warning: ksort() expects parameter 1 to be array, null given in /home/jeff/Documents/code/statusnet/classes/Memcached_DataObject.php  on line 219

Warning: Invalid argument supplied for foreach() in /home/jeff/Documents/code/statusnet/classes/Memcached_DataObject.php on line 224

Warning: assert() [function.assert]: Assertion failed in /home/jeff/Documents/code/statusnet/classes/Memcached_DataObject.php on line 241

(plus two "Cannot modify header information..." messages as a result of
the above warnings)

This change appears to fix this (although I can't really explain exactly
why).

classes/User_username.php

@@ -55,7 +55,7 @@ class User_username extends Memcached_DataObject
 
     // now define the keys.
     function keys() {
-        return array('provider_name', 'username');
+        return array('provider_name' => 'K', 'username' => 'K');
     }
 
 }